1. Technical Field
The invention relates to the repair of worn or damaged refractory linings and more particularly to a flame spraying method and composition utilizing zinc and/or magnesium oxidizable particles for use with flame spraying apparatus for the in situ repair of the linings.
2. Description of the Related Art
Coke ovens, glass furnaces, soaking pots, reheat furnaces, ladles and the like are lined with refractory brick or castings. These linings become eroded or damaged due to the stresses resulting from high temperature service.
Ideally, repairs to these linings are effected in situ while the ovens or furnaces are hot so as to eliminate cool down and heat up periods during which the ovens or furnaces are not in service.
Currently, a common method of repairing these refractory linings is to spray coat the damaged area with powdered refractory slurried with water for application. Such coatings have inherent drawbacks in that the moisture in the slurry can thermally shock the refractory area being repaired by excessive local cooling that causes spalling of the surface. In addition, the bond mechanism and the resulting wear characteristic and adherence are relatively poor so that the repair itself require frequent repairs. Such repairs are commonly done on a daily to monthly basis, depending on the type of lining being repaired and its service. For example, spray coatings on coke ovens may last only one to four weeks, and usually last less than one or two months.
Another well known technique of repairing damaged linings in situ is flame spraying the damaged area to . deposit a refractory material. In flame spraying, refractory particles and oxidizable particles are sprayed from a lance toward the damaged lining. The oxidizable particles are combusted to cause the refractory particles, or at least the surface thereof, to be molten or sintered.
U.S. Pat. Nos. 3,741,822 and 3,684,560, Swedish Patent No. 102,083, and British Patent No. GB2035524B all disclose the use of powdered metals as the oxidizable particle heat sources in the formation of shaped refractory masses. These patents teach refractory masses formed by burning highly combustible metal powders such as aluminum, silicon, and/or a magnesium in the presence of refractory oxides such as silica, alumina and/or magnesite, and oxygen as a combustion supporting gas. These processes use finely divided metal powders having a grain use size below about 50 to 100 microns. The rapid heat generated by burning the very fine particles tends to liquify or soften the entrained refractory particles as well as to soften the area being repaired.
U.S. Pat. No. 4,488,022 teaches the use of finely divided metal powders having grain size less than about 50 microns. The metal powders are a mixture of silicon and aluminum.
U.S. Patent Application Ser. No. 07/255,634, the disclosure of which is incorporated by reference, teaches the use of powdered oxidizable metal particles of chromium, aluminum or magnesium. Typically, the oxidizable metal heat sources have been chosen to be complementary to the sprayed refractory oxides and the refractories in the lining undergoing repair. Swedish Patent No. 102,083, for instance, teaches that when roasted cyanite (3AL.sub.2 O.sub.3.2SiO.sub.2) or sillimanite (AL.sub.2 O.sub.3.SiO.sub.2) are used as the refractory oxides in a flame spraying repair of linings, aluminum and silicon should be used at the same time in such proportions as correspond to the proportion in the oxides.
The art, however, has counseled against employing zinc metal as an oxidizable metal powder heat source in the repair of coke ovens and the like because metallic zinc is considered an undesirable contaminant in the ferrous metal products of the steel making and foundry industries.
Although magnesium has been suggested as a suitable heat source because metallic magnesium is not considered a contaminant, in practice, magnesium has not been employed because it is extremely reactive and tends towards hazardous back flashes when used as a heat source in flame spraying apparatuses.